If only for OTU, every empire except the vargr, for obvious reasons, has a centralized primary interstellar trade network within their borders. The game system is designed for this. Independent and neutral systems need a bit of imaginative thinking as to why they have the 'starport' they do. The local system government or business group have the tech level and resources to build one? A foreign source needs to establish such a trade center and bought the rights? Built during a time of greater power, local or foreign and now inherited?

Reading T5 and other Traveller sources, spaceports are meant to primarily service the local system's immediate needs which can include starship presence but is not the main source of trade which is part of the 'homeworld' designation.

A variant from the Traveller 1e system generation I like is using population to determine starport type, 2d6-7 + population. Makes a little more sense. T5 has highports for each type of starport based on population levels. Reduces super advanced Class A ports with highports on barren rocks.

. . . The definition of what a 'starport' is vs. what a 'spaceport' is hasn't been addressed either. . . .

I'm pretty sure that the distinction is addressed in canon, at least for the Imperium: the starport is the mainworld port recognized by the Imperial Star Port Authority, and others are spaceports. The starport isn't necessarily a single facility. It may be a single highport and a single downport, or many of each. As I mentioned, Glisten has lots of places that are recognized as starport.

Of course if one doesn't like the idea of calling a starport comprised of many distinct facilities "the starport", there's nothing wrong with calling each a starport.

This is getting closer to answering the original question, but as far as I can recall, going back to CT, there is a generalized concept of a single starport for a world, sometimes a downport, sometimes a highport, sometimes both. Practically speaking a single port is insufficient to service the needs of a planet with multi-billion population. But, as we've seen from most supplements (canon and otherwise) and the core rule books of the various editions, fleshing out a system, or even a single planet, to provide more details is almost never done.

Shipyards can exist anywhere that materials can be assembled. Planets can have multiple stations, orbital warehouses, bases at the edge of the 100D, stations orbiting moons and outer planets, etc, etc. A star system is huge place, and well-settled systems like Trin's Veil will have multiple major habitats through which traffic will pass.

We know the Imperium can operate a starport on a world and it's Imperial territory. I can recall nothing in the literature that states an incoming ship MUST land at an imperial port first, dock at an Imperial highport, or undergo Imperial customs upon entering a system. The old COACC from MT might be of use here, but it didn't really get into the idea of what/how to handle these sorts of things. Imperial law states that planets have a great deal of independence - and having more than one starport isn't a barrier to trade. The Imperial port MAY be the largest and best equipped facility, then again it may NOT be. We've nothing to go on to make this determination.

From what has been generated thus far it would appear there is no consensus on the topic.

"But, as we've seen from most supplements (canon and otherwise) and the core rule books of the various editions, fleshing out a system, or even a single planet, to provide more details is almost never done."

I have most editions and each have their own starport rules, some actually overlap. I borrow from each to make a whole.

Traveller 2e has modifiers based on population when you roll the starport making far more sense than a purely random roll at the beginning of world generation. Also has what other major facilities might be at the starport. Highguard2e expands with definitions as to when a highport space station (or a downport) is also a Starport. They also mention shipyards can be part of any facility from civilian to corporations and militaries including the empiric starport.

T5 adds determining if and when downports and highports exist and expand on what fuel types are available. Spaceports get a bit more write up about functions.

MegaTraveller says there is only one starport but other ports can accept starships and are referred to as spaceports "if only for convenience". The Referee Manual has no extra detail for starports or spaceports. The World Builder's Handbook give additional port information under Population Related Details such as determining if other primary cities also have a starport or spaceport while tech level determines if secondary cities have an H spaceport, an F space port or no spaceport. One last note, Starport classes will allow orbital cities. Essentially some world population centers are in orbit.